Method of preparing a laminar thermal imaging medium

ABSTRACT

A laminar thermal imaging medium is prepared from a first element comprising a first sheet transparent to image-forming radiation and having at least a surface zone or layer of polymeric material heat-activatable upon subjection of the thermal imaging medium to brief and intense radiation, the first element carrying a layer of porous or particulate image-forming substance having cohesivity in excess of its adhesivity for the polymeric heat-activatable layer, and, on the opposed side of the layer of porous or particulate image-forming substance from the surface zone or layer, a first layer of adhesive, and a second element comprising a second sheet carrying a second layer of adhesive. The second layer of adhesive comprises a polymeric hardenable adhesive comprising a macromolecular organic binder having acidic groups, and a photopolymerizable monomer. The first and second elements are laminated together with the first and second layers of adhesive in contact with one another, so forming a unitary laminar medium in which the hardenable adhesive remains in its unhardened condition and serves to reduce the tendency for the unitary laminar medium to delaminate on application of stresses to the medium. Only a short lag time, typically about 10-90 seconds, is required between lamination and curing of the hardenable adhesive in order to provide a strong bond between the two elements following curing.

REFERENCE TO RELATED PATENT AND APPLICATIONS

This application is a division of application Ser. No. 08/126,087, filedSep. 23, 1993, now allowed.

Application Ser. No. 07/616,853, filed Nov. 21, 1990 (now U.S. Pat. No.5,342,731) and assigned to the same assignee as the present application,describes and claims a laminar thermal imaging medium having a polymerichardenable adhesive layer, this hardenable adhesive layer being capablein its unhardened condition of reducing the tendency for the laminarthermal imaging medium to delaminate on application of physical stressesto the medium and being hardenable to a layer of sufficient hardness toprovide a durable base for the image formed by exposure of the medium.

U.S. Pat. No. 5,229,247 describes and claims a laminar thermal imagingmedium generally similar to that described in the aforementionedapplication Ser. No. 07/616,853, but in which the photohardenableadhesive layer comprises a macromolecular organic binder and aphotopolymerizable ethylenically unsaturated monomer, the imaging mediumalso comprising a polymeric elastic and non-brittle layer (hereinaftercalled the "diffusion control" layer), which resists diffusion of thephotopolymerizable ethylenically unsaturated monomer therethrough toprevent this monomer diffusing into certain other layers of the imagingmedium.

Application Ser. No. 07/923,720 (now U.S. Pat. No. 5,275,914), filedJul. 31, 1992 and assigned to the same assignee as the presentapplication, describes and claims a laminar thermal imaging mediumgenerally similar to that described in the aforementioned applicationSer. No. 07/616,853, but containing two layers of adhesive, one of theselayers comprising a polymeric hardenable adhesive comprising amacromolecular organic binder having amino or substituted amino groups,and a photopolymerizable monomer; the second adhesive layer preferablycomprises a polymer having acidic groups.

The disclosures of the aforementioned copending applications and patentare herein incorporated by reference.

BACKGROUND OF THE INVENTION

This invention relates to an adhesive composition, and to an imagingmedium comprising this adhesive composition. More particularly, thisinvention relates to a laminar imaging medium having improved resistanceto stress-induced delamination.

The provision of images by resort to media which rely upon thegeneration of heat patterns is well known. Thermally imageable media areparticularly advantageous because they can be imaged without certain ofthe requirements attending the use of silver halide based media, such asdarkroom processing and protection against ambient light. Moreover, theuse of thermal imaging materials avoids the requirements of handling anddisposing of silver-containing and other processing streams or effluentmaterials typically associated with the processing of silver halidebased imaging materials.

Various methods and systems for preparing thermally generated symbols,patterns or other images have been reported. Examples of these can befound in U.S. Pat. Nos. 2,616,961; 3,257,942; 3,396,401; 3,592,644;3,632,376; 3,924,041; 4,123,578; and 4,157,412; in United Kingdom PatentSpecification No. 1,156,996; and in International Patent Application No.PCT/US87/03249 of M. R. Etzel (published Jun. 16, 1988, as InternationalPublication No. WO 88/04237).

In the production of a thermally actuatable imaging material, it may bedesirable and preferred that an image-forming substance be confinedbetween a pair of sheets in the form of a laminate. Laminar thermalimaging materials are, for example, described in the aforementioned U.S.Pat. Nos. 3,924,041 and 4,157,412 and in the aforementionedInternational Patent Application No. PCT/US87/03249. It will beappreciated that the sheet elements of a laminar medium will affordprotection of the image-forming substance against tile effects ofabrasion, rub-off and other physical stimuli. In addition, a laminarmedium can be handled as a unitary structure, thus, obviating therequirement of bringing the respective sheets of a two-sheet imagingmedium into proper position in a printer or other apparatus used forthermal imaging of the medium material.

In the aforementioned International Patent Application No.PCT/US87/03249, there are described certain preferred embodiments of ahigh resolution thermal imaging medium, which embodiments include aporous or particulate image-forming substance (e.g., a layer of pigmentand binder) confined in a laminate structure between a pair of sheets.Upon separation of the respective sheets, after laser exposure ofportions or regions of the medium, a pair of complementary images isobtained. Among the laminar embodiments of International PatentApplication No. PCT/US87/03249 are those which include: a first sheettransparent to image-forming radiation and having at least a surfacezone or layer of polymeric material which is heat-activatable uponsubjection of the medium to brief and intense radiation; a layer ofporous or particulate image-forming substance thereon; and a secondsheet laminated and adhesively secured to the first sheet.

Upon exposure of regions or portions of the medium to brief and intenseimage-forming radiation, and conversion of absorbed energy to heat foractivation of the heat-activatable polymeric material, correspondingregions or portions of the image-forming substance are caused to be morefirmly attached or locked to the first sheet. Abutting regions orportions of image-forming substance not subjected to such image-formingradiation are, upon separation of the first and second sheets, removedby the adhesive second sheet, for formation of an image complementary tothe image on the first sheet. In preferred thermal imaging media of theaforementioned International Application, a release layer is providedover the porous or particulate image-forming substance to facilitateproper separation of the respective first and second sheets andformation of the respective complementary images.

The respective images obtained by separating the sheets of an exposedthermal imaging medium having an image-forming substance confinedtherebetween, such as a laminar image medium of the type described inthe aforementioned International Application, may exhibit substantiallydifferent characteristics. Apart from the imagewise complementary natureof these images and the relation that each may bear as a "positive" or"negative" of an original, the respective images may differ incharacter. Differences may depend upon the properties of theimage-forming substance, on the presence of and nature of additionallayer(s) in the medium, and upon the manner in which such layers failadhesively or cohesively upon separation of the sheets. Either of thepair of images may, for reasons of informational content, aesthetics orotherwise, be desirably considered the principal image. The principalimage may, however, depending upon the aforementioned properties andmodes of failure, exhibit decidedly inferior properties, such as poorerhandling characteristics, durability and abrasion resistance, ascompared with the complementary image of secondary importance.

In the production of thermal images from media having "first" and"second" sheets, of the type described in the aforementionedInternational Application, it will oftentimes be preferred, in the caseof high density images, that the principal image be that which is formedon the second sheet by transfer of non-exposed regions of coatedimage-forming substance. It will be recognized that an alternative is toform a high density image on the first (opposed) sheet by firmlyattaching the image-forming substance in areas of exposure. This is thecase because the medium provides complementary images and the desiredhigh density image can be formed on either sheet by addressing thethermally actuatable medium according to which sheet shall bear the highdensity image. Formation of a high density image on the first sheet is,however, disadvantageous since the areas of high density are created inareas of exposure (by activation of a heat-activatable image-formingzone or layer) and large areas of image-forming substance requirecorrespondingly large areas of laser actuation and energy utilizationand highly accurate laser scanning and tracking. Errors in tracking willresult in discontinuities (whiteness or voids) by failure to attachminute regions of image-forming substance and by their removal to theopposed (second) sheet upon separation of the sheets. Owing to thepsychophysical nature of human vision, minute regions of lightness(voids) against an expansive darkness tend to be noticeable.

It will, thus, be preferred that a high density image be the result ofthe transfer in non-exposed regions of coated and continuous regions ofimage-forming material (with minimal or no discontinuities or coveragevoids), rather than the result of firm connection of high densityregions of imaging material by laser-actuated operation of theheat-activatable image-forming surface, where tracking errors increasethe possibility of creating noticeable areas of discontinuity(whiteness) against the expansive high density region.

Since the formation of a preferred image in non-exposed portions ofimage-forming substance will be the result of the removal of suchsubstance from an opposed sheet with the aid of an adhesive sheet, theadhesive thereof will serve as a base for the image carried by thesheet. The nature of the adhesive, and especially its physicalproperties, may influence image quality and certain physical attributesof the image, such as the handling properties and durability of theimage. If the wrong adhesive is used, the laminar medium material mayexhibit an undesirable tendency to delaminate upon subjection to certainphysical stresses that may be created during a manufacturing operation(e.g., bending, winding, cutting or stamping operations). It may bedesirable in some instances to form a laminar medium from a pair ofendless sheet or web materials and to then cut, slit or otherwiseprovide therefrom individual film units of predetermined size. Areciprocal cutting and stamping operation used for the cutting ofindividual film units may create stress influences in the medium,causing the sheets to separate at the interface of weakestadhesivity--typically, at the interface where, by thermal actuation, thepreferential adhesion of the image-forming substance would be reversed.

In the aforementioned copending application Ser. No. 07/616,853, and inthe corresponding International Patent Application No. PCT/US91/08585(Publication No. WO 92/09441), there is disclosed an improved thermalimaging medium including a polymeric hardenable adhesive layer which inits unhardened condition serves to laminate the sheets of the mediuminto a unitary medium having a reduced tendency to delaminate uponsubjection to physical stresses and which, upon subsequent hardening(curing), provides sufficient hardness to provide improvements in imagehandling and durability; thus, the hardenable adhesive provides a firstdegree of adhesion (hereinafter called "pre-curing adhesion") when thetwo sheets are contacted with one another or shortly thereafter and asecond degree of adhesion (hereinafter called "post-curing adhesion")after the adhesive is cured. A preferred type of hardenable adhesive foruse in this medium comprises a macromolecular organic binder; aphotopolymerizable ethylenically unsaturated monomer having at least oneterminal ethylenic group capable of forming a high molecular weightpolymer by free radical-initiated, chain-propagated additionpolymerization; and a free radical-generating, additionpolymerization-initiating system activatable by actinic radiation.

This preferred type of hardenable adhesive gives good results. However,the preferred hardenable adhesive formulations described in theaforementioned application Ser. No. 07/616,853, which comprise apolyfunctional acrylate monomer admixed with a methacrylate copolymer,require a substantial lag time (the period between the lamination of thetwo sheets and the curing of the hardenable adhesive) to ensure thatafter curing the two sheets adhere sufficiently to one another. Thissubstantial lag time, which is of the order of tens of minutes, ispresumably required because it is necessary for the polyfunctionalacrylate monomer to diffuse into an adjacent layer of the imaging mediumin order to provide sufficient post-curing adhesion. In some cases, asfor example where it is desired to carry out curing of the adhesive "inline" with the lamination (i.e., when the medium is to move continuouslyat a substantial speed along a production line from the lamination tothe curing operations, perhaps via intervening cutting or otherstations), the need for a substantial lag time in order to developpost-curing adhesion is disadvantageous since the production line mustbe modified to provide a long travel for the medium between thelamination station and the curing station, and providing such a longtravel will normally involve the provision of numerous extra rollers inthe production line, thus increasing the cost, size and powerconsumption of the line.

In the aforementioned application Ser. No. 07/923,720, there isdescribed and claimed an imaging medium and process generally similar tothat described in the aforementioned application Ser. No. 07/616,853 butin which the lag time necessary to develop substantial post-curingadhesion can be substantially reduced. As already mentioned, the imagingmedium of application Ser. No. 07/923,720 contains two layers ofadhesive, one of these layers comprising a polymeric hardenable adhesivecomprising a macromolecular organic binder having amino or substitutedamino groups, and a photopolymerizable monomer; the second adhesivelayer preferably comprises a polymer having acidic groups. During thepreparation of the imaging medium, one of the two layers of adhesive isapplied adjacent the layer of image-forming substance, while the otherlayer is applied to the second sheet, so that when the first and secondsheets are laminated together, the two adhesive layers come intocontact. In a preferred form of this imaging medium, it is the adhesivelayer on the second sheet which contains the macromolecular organicbinder because then the adhesive layer adjacent the layer ofimage-forming substance can be chosen so that it fulfills the functionof the diffusion control layer of the aforementioned U.S. Pat. No.5,229,247, and prevents or reduces diffusion of the photopolymerizablemonomer into the layer of image-forming substance, thus avoiding theneed for a separate diffusion control layer.

The imaging medium of the aforementioned application Ser. No. 07/923,720enables the lag time to be reduced to about 10 to about 30 seconds.However, the macromolecular organic binder having amino or substitutedamino groups used in this medium should not come into contact with apolymer which is adversely affected by base, and there are certainpolymers, which persons skilled in the art may wish to use as theadhesive/diffusion control layer, which are susceptible to undesirablechanges in the presence of base. In particular, as noted in theaforementioned U.S. Pat. No. 5,229,247, polymers and copolymers ofvinylidene chloride have properties which render them very suitable foruse in diffusion control layers. However, polymers and copolymers ofvinylidene chloride are susceptible to dehydrochlorination by baseand/or heat and/or ultraviolet radiation, and the products resultingfrom such dehydrochlorination may cause an undesirable tint in theimaged medium.

This invention relates to a modified form of the imaging mediumdisclosed in the aforementioned application Ser. No. 07/923,720. Thismodified imaging medium allows both a short lag time and the use in anadhesive/diffusion control layer of polymers which are adverselyaffected by base.

SUMMARY OF THE INVENTION

Accordingly, this invention provides a method of preparing a laminarthermal imaging medium. This method comprises the steps of:

providing a first element comprising a first sheet transparent toimage-forming radiation and having at least a surface zone or layer ofpolymeric material heat-activatable upon subjection of the thermalimaging medium to brief and intense radiation, the first elementcarrying a layer of porous or particulate image-forming substance havingcohesivity in excess of its adhesivity for the polymericheat-activatable layer, and, on the opposed side or the layer of porousor particulate image-forming substance from the surface zone or layer, afirst layer of adhesive;

providing a second element comprising a second sheet carrying a secondlayer of adhesive;

the second layer of adhesive comprising a polymeric hardenable adhesivecomprising a macromolecular organic binder having acidic groups, and aphotopolymerizable monomer;

laminating the first and second elements together with the first andsecond layers of adhesive in contact with one another and with the firstand second sheets outermost and forming a unitary laminar medium inwhich the hardenable adhesive remains in its unhardened condition andserves to reduce the tendency for the unitary laminar medium todelaminate on application of stresses to the medium; and

subjecting the unitary laminar medium to actinic radiation effective tocause polymerization of the photopolymerizable monomer, thus hardeningthe hardenable adhesive into a durable polymeric layer.

This invention also provides a laminar thermal imaging medium,actuatable in response to intense image-forming radiation for productionof an image, the laminar medium comprising in order:

a first sheet transparent to the image-forming radiation and having atleast a surface zone or layer of polymeric material heat-activatableupon subjection of the thermal imaging medium to brief and intenseradiation;

a layer of porous or particulate image-forming substance havingcohesivity in excess of its adhesivity for the polymericheat-activatable layer;

a first layer of adhesive affixed, directly or indirectly, to the layerof porous or particulate image-forming substance;

a second layer of adhesive adhered to the first layer of adhesive; and

a second sheet covering the layer of porous or particulate image-formingsubstance and adhered, via the first and second layers of adhesive, tothe layer of image-forming substance, the second sheet, upon separationof the first and second sheets after exposure to the intense radiation,being adapted to the removal therewith of unexposed portions of theimage-forming substance;

the second layer of adhesive comprising a polymeric hardenable adhesivecomprising a macromolecular organic binder having acidic groups, and aphotopolymerizable monomer, the hardenable adhesive layer being capablein its unhardened condition of reducing the tendency for the laminarthermal imaging medium to delaminate on application of physical stressesto the medium and being hardenable to a layer of sufficient hardness toprovide a durable base for the image,

the thermal imaging medium being capable of absorbing the radiation ator near the interface of the surface zone or layer with the layer ofporous or particulate image-forming substance, and converting theabsorbed radiation into heat sufficient in intensity to heat activatethe surface zone or layer, such that upon cooling exposed portions ofthe layer of porous or particulate image-forming substance are morefirmly attached to the first sheet,

the thermal imaging medium being adapted to image formation by imagewiseexposure of portions of the thermal imaging medium to radiation ofsufficient intensity to attach exposed portions of the layer of porousor particulate image-forming substance firmly to the first sheet, and byremoval to the second sheet, upon separation of the first and secondsheets after the imagewise exposure, of unexposed portions of the layerof porous or particulate image-forming substance, thereby to providefirst and second images, respectively, on the first and second sheets.

This invention extends to this medium in both its uncured and its curedforms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of a preferred laminar thermallyactuatable imaging medium of the present invention; and

FIG. 2 is a schematic cross-section similar to FIG. 1 but showing themedium in a state of partial separation after thermal imaging.

DETAILED DESCRIPTION OF THE INVENTION

As already mentioned, the method of the present invention uses a firstelement comprising a first sheet transparent to image-forming radiationand having a surface zone or layer of polymeric materialheat-activatable upon subjection of the thermal imaging medium to briefand intense radiation. The first element also comprises a layer ofporous or particulate image-forming substance having cohesivity inexcess of its adhesivity for the polymeric heat-activatable layer, and,on the opposed side of the layer of porous or particulate image-formingsubstance from the surface zone or layer, a first layer of adhesive(hereinafter called the "first adhesive layer"). The present method alsouses a second element comprising a second sheet carrying a second layerof adhesive (hereinafter called the "second adhesive layer"). The secondadhesive layer comprises a polymeric hardenable adhesive comprising amacromolecular organic binder having acidic groups, and aphotopolymerizable monomer. The first and second elements are laminatedtogether with the first and second adhesive layers in contact with oneanother and with the first and second sheets outermost, so forming aunitary laminar medium in which the hardenable adhesive remains in itsunhardened condition; this unhardened adhesive layer serves to reducethe tendency for the unitary laminar medium to delaminate on applicationof stresses to the medium. Later, the medium is subjected to actinicradiation effective to cause polymerization of the photopolymerizablemonomer, thus hardening the hardenable adhesive into a durable polymericlayer.

So far as the development of post-curing adhesion is concerned, eitherof the two adhesive layers could contain the macromolecular organicbinder having acidic groups. However, for practical reasons thismacromolecular organic binder is present in the second adhesive layer.As explained above and in the aforementioned U.S. Pat. No. 5,229,247,when hardenable adhesives containing a macromolecular binder and apolymerizable monomer are incorporated into thermal imaging media, themonomer tends to diffuse into other layers of the media and affect theproperties of the media. Thus, if such a hardenable adhesive isincorporated into the first element of an imaging medium of the presentinvention prior to lamination of the first and second elements,polymerizable monomer may diffuse into the other layers of the firstelement, with adverse effects on the functioning of the final imagingmedium. If, on the other hand, the hardenable adhesive is incorporatedinto the second element of the imaging medium (this second elementtypically comprises only the second adhesive layer and the secondsheet), prior to lamination of the first and second elements thepolymerizable monomer cannot diffuse into any other layers where itcould affect the performance of the final imaging medium.

With the monomer present in the second adhesive layer, the firstadhesive layer provides some resistance to diffusion of the monomertherethrough (thereby acting as a diffusion control layer) afterlamination, and thus reduces the tendency for the monomer to reach thelayer of image-forming substance, or another layer where its presencemight adversely affect the performance of the imaging medium. Tominimize diffusion of polymerizable monomer into the layer ofimage-forming substance after lamination but before curing of theadhesive, it is, as already mentioned, desirable that the material ofthe first adhesive layer be chosen to resist diffusion of thephotopolymerizable monomer therethrough. Examples of materials suitablefor use in the first adhesive layer include halogen-containing polymers,for instance polymers comprising repeating units derived from vinylidenechloride, such as that sold under the tradename Daran SL-158 byHampshire Chemical Corporation, of 55 Hayden Road, Lexington Mass.02173; this material is stated by the manufacturer to be a copolymer ofvinylidene chloride and methyl acrylate. As already noted, the use of amacromolecular organic binder containing acidic groups in the secondadhesive layer of the present medium enables such vinylidene chloridepolymers and copolymers to be used in the first adhesive layer, despitesusceptibility of such polymers and copolymers to base-induceddehydrohalogenation. While this invention does not exclude thepossibility that a diffusion control layer, separate from the firstadhesive layer, could be incorporated into the imaging medium betweenthe layer of image-forming substance and the first adhesive layer, ingeneral the provision of such a separate diffusion control layer isunnecessary.

The present imaging medium typically only requires a lag time of about10 to 90 seconds between lamination of the first and second elements andcuring of the hardenable adhesive, and thus will normally be used in aproduction process in which curing of the hardenable adhesive occurs inline with the lamination step, so that a lag time of only about 10seconds to a few minutes elapses. Thus, in the method of the presentinvention, there is little time for the photopolymerizable monomer todiffuse into layers where its presence might adversely affect theperformance of the imaging medium.

In thermal imaging media of the type described in the aforementionedapplication Ser. No. 07/616,853 and the aforementioned U.S. Pat. No.5,229,247, the cured adhesive layer may, depending upon the hardnessthereof, fracture or shatter, or separate from the second sheet, onapplication of stress, as for example where an image formed upon thesecond sheet is flexed or distorted. Also, in some media of this type,if a single sheet or a pack of sheets of the medium is dropped sharplyon one edge, the structure of the imaging medium is disrupted and theimaging performance of the medium is no longer satisfactory. It has beenfound that the use of first and second adhesive layers, the second ofwhich contains acidic groups, in accordance with the present invention,substantially reduces these problems.

In the macromolecular organic binder of the present medium, the acidicgroups are preferably carboxylic acid groups; desirably the binder is apolymer having repeating units derived from acrylic or methacrylic acid,for example a copolymer of acrylic or methacrylic acid with one or moreacrylate or methacrylate esters. This type of copolymer desirably has aglass transition temperature in the range of from about -10° to about+50° C., preferably about -10° to about +30° C. An especially preferredgroup of copolymers of this type comprise from 0 to about 20 parts byweight of butyl acrylate; from about 70 to about 99 parts by weight ofbutyl methacrylate; and from about 1 to about 10 parts by weight ofacrylic acid. Alternatively, the macromolecular organic binder may be apolymer having repeating units derived from β-carboxyethyl acrylate ormaleic acid.

It some cases, it may be convenient to form the acid groups of themacromolecular organic binder after the binder itself is prepared, i.e.,the binder could be prepared having precursor groups thereon, andthereafter the binder could be treated to convert these precursor groupsinto the acidic groups. In particular, where the final binder is tocontain repeating units derived from a dibasic acid (for example, maleicacid) which forms an anhydride, it may be convenient to first form thebinder using the non-acidic anhydride. Dispersion of theanhydride-containing form of the binder in an aqueous medium, followedby coating of the second sheet with this medium and drying to form thesecond adhesive layer, under conditions conventionally employed to formimaging media, will normally hydrolyze the anhydride repeating units ofthe binder to the free acid form without any additional steps.

The photopolymerizable monomer used in the present imaging mediumdesirably comprises a di- or higher functional acrylate or methacrylate,preferably a tri- or higher functional acrylate or methacrylate, aspecific preferred monomer being trimethylolpropane triacrylate.

For optimum performance of the present imaging medium, the properties ofboth the uncured and cured forms of the hardenable adhesive layer shouldbe carefully chosen. Prior to curing, the hardenable adhesive layerserves to hold the first and second elements of the imaging mediumtogether and to reduce the effects of stress on the medium, while aftercuring the hardenable adhesive layer affects the mechanical propertiesof the medium, and after imaging acts as a base on which an image rests.It has been found experimentally that increasing the level of acidicgroups in the hardenable adhesive not only increases the rapiddevelopment of post-curing adhesion (thus reducing the lag timerequired) but also increases the strength of the cured adhesive, therebyincreasing the resistance of an image formed thereon to removal ordistortion of the areas of the image containing the porous orparticulate image-forming substance; if the cured hardenable adhesive istoo soft, mechanical stress, such as that caused by handling of theimage, may cause removal of the image-forming substance or distortion ofthe pels of the image. The resistance of the image to removal ordistortion is also affected by the glass transition temperature of themacromolecular organic binder used in the hardenable adhesive and theamount of photopolymerizable monomer used therein; the stiffness of thecured hardenable adhesive tends to increase as the glass transitiontemperature or the concentration of photopolymerizable monomerincreases. However, care should be taken not to increase the stiffnessof the cured hardenable adhesive too far, since an excessively stiffcured hardenable adhesive may increase the susceptibility of the imagingmedium to the aforementioned shattering problem.

In FIG. 1, there is shown a preferred laminar imaging medium (generallydesignated 10) of the present invention suited to production of a pairof high resolution images, shown in FIG. 2 as images 10a and 10b in apartial state of separation. Thermal imaging medium 10 includes a firstelement in the form of a first sheet-like or web material 12 (comprisingsheet material 12a, stress-absorbing layer 12b and heat-activatable zoneor layer 12c) having superposed thereon, and in order, porous orparticulate image-forming layer 14, release layer 16, firstadhesive/diffusion control layer 18, second, hardenable polymericadhesive layer 20 and second sheet-like or web material 22.

Upon exposure of the medium 10 to infra-red radiation, exposed portionsof image-forming layer 14 are more firmly attached to web material 12,so that, upon separation of the respective sheet-like materials, asshown in FIG. 2, a pair of images, 10a and 10b, is provided. The natureof certain of the layers of preferred thermal imaging medium material 10and their properties are importantly related to the manner in which therespective images are formed and partitioned from the medium afterexposure. The functioning of hardenable adhesive layer 20 is importantto the reduction of undesired delamination at the interface betweenheat-activatable zone or layer 12c and porous or particulateimage-forming layer 14 of the preferred thermal imaging medium shown inFIG. 1. The various layers of medium material 10 are described in detailhereinafter.

Web material 12 comprises a transparent material through which imagingmedium 10 can be exposed to radiation. Web material 12 can comprise anyof a variety of sheet-like materials, although polymeric sheet materialswill be especially preferred. Among preferred sheet materials arepolystyrene, poly(ethylene terephthalate), polyethylene, polypropylene,poly(vinyl chloride), polycarbonate, poly(vinylidene chloride),cellulose acetate, cellulose acetate butyrate and copolymeric materialssuch as the copolymers of styrene, butadiene and acrylonitrile,including poly(styrene-co-acrylonitrile). An especially preferred sheetmaterial from the standpoints of durability, dimensional stability andhandling characteristics is poly(ethylene terephthalate), commerciallyavailable, for example, under the tradename Mylar, of E. I. du Pont deNemours & Co., Wilmington Del., or under the tradename Kodel, of EastmanKodak Company, Rochester, N.Y.

The stress-absorbing layer 12b is as described in U.S. Pat. No.5,200,297 and the corresponding International Patent Application No.PCT/US91/08604 (Publication No. WO 92/09443), and comprises a polymericlayer capable of absorbing physical stresses applied to the imagingmedium 10. The stress-absorbing layer 12b provides added protectionagainst delamination of the medium 10 when rigorous physical stressesare applied thereto, and is desirably formed from a compressible orelongatable polyurethane. The stress-absorbing layer 12b is optional andmay sometimes be omitted, depending upon the second adhesive layer 20used and the stresses to which the medium 10 will be subjected.

Heat-activatable zone or layer 12c provides an essential function in theimaging of medium 10 and comprises a polymeric material which is heatactivatable upon subjection of the medium to brief and intenseradiation, so that, upon rapid cooling, exposed portions of the surfacezone or layer 12c are firmly attached to porous or particulateimage-forming layer 14. If desired, when the stress-absorbing layer 12bis omitted, surface zone 12c can be a surface portion or region of webmaterial 12, in which case, layers 12a and 12c will be of the same orsimilar chemical composition. In general, it is preferred that layer 12ccomprise a discrete polymeric surface layer on sheet material 12a orstress-absorbing layer 12b. Layer 12c desirably comprises a polymericmaterial having a softening temperature lower than that of sheetmaterial 12a, so that exposed portions of image-forming layer 14 can befirmly attached to web material 12. A variety of polymeric materials canbe used for this purpose, including polystyrene,poly(styrene-co-acrylonitrile), poly(vinyl butyrate), poly(methylmethacrylate), polyethylene and poly(vinyl chloride).

The employment of a thin heat-activatable layer 12c on a substantiallythicker and durable sheet material 12a permits desired handling of theweb material and desired imaging efficiency. The use of a thinheat-activatable layer 12c facilitates the concentration of heat energyat or near the interface between layers 12c and image-forming layer 14and permits optimal imaging effects and reduced energy requirements. Itwill be appreciated that the sensitivity of layer 12c to heat activation(or softening) and attachment or adhesion to layer 14 will depend uponthe nature and thermal characteristics of layer 12c and upon thethickness thereof.

Stress-absorbing layer 12b can be provided on sheet material 12a by themethods described in the aforementioned U.S. Pat. No. 5,200,297 andInternational Patent Application No. PCT/US91/08604. Heat-activatablelayer 12c can be provided by resort to known coating methods. Forexample, a layer of poly(styrene-co-acrylonitrile) can be applied to aweb of poly(ethylene terephthalate) by coating from an organic solventsuch as methylene chloride. The desired handling properties of webmaterial 12 will be influenced mainly by the nature of sheet material12a itself, since layers 12b and 12c will be coated thereon as thinlayers. The thickness of web material 12 will depend upon the desiredhandling characteristics of medium 10 during manufacture, imaging andany post-imaging steps. Thickness will also be dictated in part by theintended use of the image to be carried thereon and by exposureconditions, such as the wavelength and power of the exposing source.Typically, web material 12 will vary in thickness from about 0.5 to 7mil (about 13 to 178 μm). Good results are obtained using, for example,a sheet material 12a having a thickness of about 1.5 to 1.75 mils (38 to44 μm). Stress-absorbing layer 12b will typically have a thickness inthe range of about 1 to 4 μm, while layer 12c will typically be a layerof poly(styrene-co-acrylonitrile) having a thickness of about 0.1 to 5μm.

Heat-activatable layer 12c can include additives or agents providingknown beneficial properties. Adhesiveness-imparting agents,plasticizers, adhesion-reducing agents, or other agents can be used.Such agents can be used, for example, to control adhesion between layers12c and 14, so that undesired separation at the interface thereof isminimized during manufacture of laminar medium 10 or during use thereofin a thermal imaging method or apparatus. Such control also permits themedium, after imaging and separation of sheet-like web materials 12 and22, to be partitioned in the manner shown in FIG. 2.

Image-forming layer 14 comprises an image-forming substance deposited onto heat-activatable zone or layer 12c as a porous or particulate layeror coating. Layer 14, also called a colorant/binder layer, can be formedfrom a colorant material dispersed in a suitable binder, the colorantbeing a pigment or dye of any desired color, and preferably beingsubstantially inert to the elevated temperatures required for thermalimaging of medium 10. Carbon black is a particularly advantageous andpreferred pigment material. Preferably, the carbon black material willcomprise particles having an average diameter of about 0.01 to 10 μm.Although the description herein will refer principally to carbon black,other optically dense substances, such as graphite, phthalocyaninepigments and other colored pigments can be used. If desired, substanceswhich change their optical density upon subjection to temperatures asherein described can also be employed.

The binder for the image-forming substance or layer 14 provides a matrixto form the porous or particulate substance thereof into a cohesivelayer and serves to adhere layer 14 to heat-activatable zone or layer12c. In general, it will be desired that image-forming layer 14 beadhered to surface zone or layer 12c sufficiently to prevent accidentaldislocation either during the manufacture of medium 10 or during the usethereof. Layer 14 should, however, be separable (in non-exposed regions)from zone or layer 12c, after imaging and separation of webs 12 and 22,so that partitioning of layer 14 can be accomplished in the manner shownin FIG. 2.

Image-forming layer 14 can be conveniently deposited on to surface zoneor layer 12c, using any of a number of known coating methods. Accordingto a one embodiment, and for ease in coating layer 14 on to zone orlayer 12c, carbon black particles are initially suspended in an inertliquid vehicle, with a binder or dispersant, and the resultingsuspension or dispersion is uniformly spread over heat-activatable zoneor layer 12c. On drying, layer 14 is adhered as a uniform image-forminglayer on the surface thereof. It will be appreciated that the spreadingcharacteristics of the suspension can be improved by including asurfactant, such as ammonium perfluoroalkyl sulfonate, non-ionicethoxylate or the like. Other substances, such as emulsifiers, can beused or added to improve the uniformity of distribution of the carbonblack in its suspended state and, thereafter, in its spread and drystate. Layer 14 can range in thickness and typically will have athickness of about 0.1 to about 10 μm. In general, it is preferred fromthe standpoint of image resolution, that a thin layer 14 be employed.Layer 14 should, however, be of sufficient thickness to provide desiredand predetermined optical density in the images prepared from imagingmedium 10.

Suitable binder materials for image-forming layer 14 include gelatin,poly(vinyl alcohol), hydroxyethyl cellulose, gum arabic, methylcellulose, polyvinylpyrrolidone, polyethyloxazoline, polystyrene latexand poly(styrene-comaleic anhydride). The ratio of pigment (e.g., carbonblack) to binder can be in the range of from 40:1 to about 1:2 on aweight basis. Preferably, the ratio of pigment to binder will be in therange of from about 4:1 to about 10:1. A preferred binder material for acarbon black pigment material is poly(vinyl alcohol).

If desired, additional additives or agents can be incorporated intoimage-forming layer 14. Thus, submicroscopic particles, such as chitin,polytetrafluoroethylene particles and/or polyamide can be added tocolorant/binder layer 14 to improve abrasion resistance. Such particlescan be present, for example, in amounts of from about 1:2 to about 1:20,particles to layer solids, by weight.

For the production of images of high resolution, it will be essentialthat image-forming layer 14 comprise materials that permit fracturethrough the thickness of the layer and along a direction substantiallyorthogonal to the interface between surface zone or layer 12c andimage-forming layer 14, i.e., substantially along the direction ofarrows 24, 24', 26, and 26', shown in FIG. 2. It will be appreciatedthat, in order for images 10a and 10b to be partitioned in the mannershown in FIG. 2, image-forming layer 14 will be orthogonally fracturableas described above and will have a degree of cohesivity in excess of itsadhesivity for heat-activatable zone or layer 12c. Thus, on separationof webs 12 and 22 after imaging, layer 14 will separate in non-exposedareas from heat-activatable layer 12c and remain in exposed areas asporous or particulate portions 14a on web 12. Layer 14 is an imagewisedisruptible layer owing to the porous or particulate nature thereof andthe capacity for the layer to fracture or break sharply at particleinterfaces.

The release layer 16 shown in FIG. 1 is included in thermal imagingmedium 10 to facilitate separation of images 10a and 10b according tothe mode shown in FIG. 2. As described above, regions of medium 10subjected to radiation become more firmly secured to heat-activatablezone or layer 12c by reason of the heat activation of the layer by theexposing radiation. Non-exposed regions of layer 14 remain only weaklyadhered to heat-activatable zone or layer 12c and are carried along withsheet 22 on separation of sheets 12 and 22. This is accomplished by theadhesion of layer 14 to heat-activatable zone or layer 12c, innon-exposed regions, being less than: (a) the adhesion between layers 14and 16; (b) the adhesion between layers 16 and 18; (c) the adhesionbetween layers 18 and 20; (d) the adhesion between layer 20 and sheet22; and (e) the cohesivity of layers 14, 16, 18 and 20. The adhesion ofsheet 22 to porous or particulate layer 14, through layers 16, 18 and20, while sufficient to remove non-exposed regions of porous andparticulate layer 14 from heat-activatable zone or layer 12c, iscontrolled, in exposed areas, by release layer 16 so as to preventremoval of firmly attached exposed portions 14a of layer 14 (attached toheat-activated zone or layer 12c by exposure thereof).

Release layer 16 is designed such that its cohesivity and its adhesionto either first adhesive/diffusion control layer 18 or porous orparticulate layer 14 is less, in exposed regions, than the adhesion oflayer 14 to heat-activated zone or layer 12c. The result of theserelationships is that release layer 16 undergoes an adhesive failure inexposed areas at the interface between layers 16 and 18, or at theinterface between layers 14 and 16; or, as shown in FIG. 2, a cohesivefailure of layer 16 occurs, such that portions (16b) are present inimage 10b and portions (16a) are adhered in exposed regions to porous orparticulate portions 14a. Portions 16a of release layer 16 serve toprovide surface protection for the image areas of image 10a againstabrasion and wear.

Release layer 16 can comprise a wax, wax-like or resinous material.Microcrystalline waxes, for example, high density polyethylene waxesavailable as aqueous dispersions, can be used for this purpose. Othersuitable materials include carnauba, beeswax, paraffin wax and wax-likematerials such as poly(vinyl stearate), poly(ethylene sebacate), sucrosepolyesters, polyalkylene oxides and dimethylglycol phthalate. Polymericor resinous materials such as poly(methyl methacrylate) and copolymersof methyl methacrylate and monomers copolymerizable therewith can beemployed. If desired, hydrophilic colloid materials, such as poly(vinylalcohol), gelatin or hydroxyethyl cellulose can be included as polymerbinding agents.

Resinous materials, typically coated as latices, can be used and laticesof poly(methyl methacrylate) are especially useful. Cohesivity of layer16 can be controlled so as to provide the desired and predeterminedfracturing. Waxy or resinous layers which are disruptible and which canbe fractured sharply at the interfaces of particles thereof can be addedto the layer to reduce cohesivity. Examples of such particulatematerials include, silica, clay particles and particles ofpolytetrafluoroethylene.

As discussed above, the first adhesive/diffusion control layer 18 servesboth to adhere the first and second elements together and to reducediffusion of the polymerizable monomer into the various layers of thefirst element after the first and second layers are laminated together.On contact with the second adhesive layer 20 (discussed in detailbelow), layer 18 serves to develop rapidly substantial pre-curing andpost-curing adhesion to the second adhesive layer 20, thus securing thefirst and second elements together to form the unitary laminar imagingmedium 10. A specific preferred copolymer for use in layer 18 is thatavailable as Daran SL-158 from Hampshire Chemical Corporation, of 55Hayden Road, Lexington Mass. 02173.

Layer 18 can be provided on release layer 16 by any of a variety ofknown coating methods. In the case of a preferred polyvinylidenechloride diffusion control material, a latex of the polymer can becoated on to an element comprising sheet 12 and layers 14 and 16. Thelayer is then dried and laminated to sheet 22 carrying layer 20.

While a principal function of layer 18 is to provide adhesion anddiffusion control properties, other properties of layer 18 provideadditional benefits in medium 10. As can be seen from FIG. 2, and fromimage 10b in particular, after imaging layers 20 and 18, and portions ofrelease layer 16, serve as a base for portions 14b of image-formingsubstance. The handling properties of image 10b and the durabilitythereof will, thus, be influenced by the nature of each such layer, bythe adhesion between the respective interfaces of such layers and, inparticular, by the adhesion of layer 20 to support 22.

The thickness of layer 18 can vary with the particular nature andconstituency thereof and with the diffusion control and elasticqualities thereof. In general, layer 18 will have a thickness of about1.5 to about 5 μm, and preferably about 3 μm.

The second adhesive layer 20 of imaging medium 10 comprises a hardenableadhesive layer which is capable of protecting the medium againststresses that would create a delamination of the medium, typically atthe interface between zone or layer 12c and image-forming layer 14. Thephysical stresses, which tend to promote delamination and which can bealleviated by hardenable layer 20, can vary, and include stressescreated by bending the laminar medium and stresses created by winding,unwinding, cutting, slitting or stamping operations. Since layer 20 canvary in composition, it will be appreciated that a particular adhesivemay, for example, provide protection of the medium against delaminationpromoted by bending of the medium, while providing little or noprotection against delamination caused, for example, by a slitting orstamping-and-cutting operation, or vice versa.

As already mentioned, imaging medium 10 is normally prepared by thelamination of first and second sheet-like web elements or components,the first element or component comprising web material 12 carryingimage-forming layer 14, release layer 16 and first adhesive layer 18,while the second element comprises second adhesive layer 20 and secondweb material 22. The two elements can be laminated under pressure, andoptionally under heating conditions, to provide the unitary and laminarthermally actuatable imaging medium 10 of the invention.

The imaging medium 10 may be subjected to a variety of handling and/orcutting procedures before and/or after curing of the second adhesivelayer 20. The lamination of the first and second elements is typicallyconducted on endless webs of the two elements. Following this laminationof endless webs, individual sheets of predetermined size suited, forexample, to stacking in a cassette for feeding into a printer apparatuscan be prepared from the endless web by die cutting or similar methods.Because of the high shear stresses involved in die cutting, such diecutting will typically be performed before curing of the second adhesivelayer 20, so that the uncured adhesive layer 20 can serve to eliminateor minimize delamination of the medium 10 caused by the shear stressesto which the medium 10 is exposed during die cutting.

While applicants do not wish to be bound by any particular theory ormechanism in explanation of the manner in which the second adhesivelayer 20 serves to minimize stress-induced delamination of the mediummaterial, it is believed that this layer 20 may serve to absorb physicalstresses applied to medium and thereby reduce the incidence ofdelamination. Alternatively, layer 18 may serve to distribute stressesthroughout the layer or otherwise prevent applied stresses from beingtransmitted through the medium and from causing delamination.

Alternatively, the second adhesive layer 20 may first be cured andthereafter be subjected to cutting operations. Such post-curing cuttingis best effected by techniques, such as slitting, which do not placegreat stress upon the medium. For example, the medium 10 could belaminated and cured in-line and thereafter subjected to slitting to trimthe edges of the medium before the cured medium is wound on to a roll,or cut into individual sheets.

As already mentioned, the medium of the present invention has theimportant advantage that only a short lag time, typically about 10 toabout 90 seconds, is required between lamination of the two elements ofthe imaging medium and curing in order to develop strong post-curingadhesion. Accordingly, curing of the medium can conveniently be effectedin line without the need to provide a very long travel for the mediumbetween the lamination station and the curing station.

Cutting of the present imaging medium can be effected before or aftercuring; in some cases, the choice between pre-curing and post-curingcutting will be determined by the cutting technique employed. If it isdesired to cut before curing, such cutting can be performed eitherin-line with lamination and curing, or off line.

Preferred macromolecular binders for use in the second adhesive layer 20have already been discussed above. Suitable photopolymerizableethylenically unsaturated monomers for such compositions include thedi-, tri- and higher functional acrylates, such as the aforementionedacrylate and methacrylate esters of polyhydric alcohols (e.g.,pentaerythritol triacrylate and trimethylolpropane triacrylate, thelatter being the especially preferred monomer for use in the presentmethod and imaging medium). Other suitable monomers include ethyleneglycol diacrylate or dimethacrylate or mixtures thereof; glyceroldiacrylate or triacrylate; urethane acrylates; and epoxy acrylates. Ingeneral, photopolymerizable monomers which provide tack in suchcompositions or which serve to plasticize the macromolecular binder willbe preferred.

Those skilled in the art of photopolymerization will be aware that mostphotopolymerizable monomers require the presence of a photoinitiator forpolymerization (curing) of the monomer to occur, and thus typically thesecond adhesive layer 20 will contain such a photoinitiator. Thephotoinitiators and concentrations thereof required with variousphotopolymerizable monomers are well known to those skilled in the art,and the conventional types and concentrations of photoinitiators can beused in the second adhesive layer 20. Thus, the specific preferredphotopolymerizable monomer trimethylolpropane triacrylate requires thepresence of a free-radical generating photoinitiator, for example thatsold commercially under the tradename Irgacure 651 by Ciba-Geigy. Toprevent premature curing of the hardenable adhesive, it may be desirableto include a small amount of a free radical inhibitor, for example aphenol.

In general, second adhesive layer 20 can be coated as a low viscositysolution and then dried to a highly viscous coating. Anti-oxidants canbe included, if desired. Thickeners, binders and coating aids can beincluded to control viscosity and facilitate coating to a uniform andadhesive layer. Tack-promoting and plasticizing agents can be includedfor their known properties.

Photohardening of second adhesive layer 20 can be accomplished in knownmanner by polymerization, using conventional sources of ultravioletradiation such as carbon arc lamps, commercially available ultra-violetelectrodeless bulbs (for example, "D" and "H" bulbs sold by Fusion UVCuring Systems, 7600 Standish Place, Rockville Md. 20855-2798), xenonlamps and medium pressure mercury lamps. The choice of a suitableirradiating source for hardening will also depend on the thickness ofthe layer to be hardened.

The thickness of the second adhesive layer can vary and, in general,will be in the range of from 0.1 to 50 μm. A preferred range ofthickness is from 0.5 to 20 μm.

As is known in the art, photopolymerization systems are often sensitiveto atmospheric oxygen. The use of photopolymerizable compositions asdescribed above and which are sensitive to oxygen can be used toadvantage. Individually cut units of medium 10 tend, at the edge regionsof second adhesive layer 20 about the perimeter of the laminar medium,to be incompletely polymerized and to retain a degree of softness whichreduces the tendency for the medium to delaminate.

The use of hardenable second adhesive layer 20 in medium 10 isadvantageous from the standpoint of permitting lamination of thecomponents thereof without the requirement of elevated temperatures thatmay have an adverse influence on other layers or components of themedium. While heat and pressure can be used to effect the lamination,pressing of the components without heat can be used to provide thelamination. The use of a hardenable layer 20 that can be cured underambient room conditions reduces the required dwell time to achievelamination and increases manufacturing efficiency.

Upon curing of second adhesive layer 20, medium material 10 is ready forimaging. Attachment of weakly adherent image-forming layer 14 toheat-activatable zone or layer 12c in areas of exposure is accomplishedby (a) absorption of radiation within the imaging medium; (b) conversionof the radiation to heat sufficient in intensity to heat activate zoneor layer 12c; and (c) cooling to more firmly join exposed regions orportions of layer 14 to heat-activatable zone or layer 12c. Thermalimaging medium 10 is capable of absorbing radiation at or near theinterface of layer 14 with heat-activatable zone or layer 12c. This isaccomplished by using layers in medium 10 which by their nature absorbradiation and generate the requisite heat for desired thermal imaging,or by including, in at least one of the layers, an agent capable ofabsorbing radiation of the wavelength of the exposing source.Infrared-absorbing dyes can, for example, be suitably employed for thispurpose.

If desired, porous or particulate image-forming substance 14 cancomprise a pigment or other colorant material such as carbon blackwhich, as is more completely described hereinafter, is absorptive ofexposing radiation and which is known in the thermographic imaging fieldas a radiation-absorbing pigment. Because a secure bonding or joining isdesired at the interface of layer 14 and heat-activatable zone or layer12c, it may be preferred in some instances that a radiation-absorbingsubstance be incorporated into either or both of image-forming layer 14and heat-activatable zone or layer 12c.

Suitable radiation-absorbing substances in layers 14 and/or 12c, forconvening radiation into heat, include carbon black, graphite or finelydivided pigments such as the sulfides or oxides of silver, bismuth ornickel. Dyes such as the azo dyes, xanthene dyes, phthalocyanine dyes oranthraquinone dyes can also be employed for this purpose. Especiallypreferred are materials which absorb efficiently at the particularwavelength of the exposing radiation. Infrared dyes which absorb in theinfrared-emitting regions of lasers which are desirably used for thermalimaging are especially preferred. Suitable examples ofinfrared-absorbing dyes for this purpose include thealkylpyrylium-squarylium dyes, disclosed in U.S. Pat. No. 4,508,811, andincluding 1,3-bis(2,6-di-t-butyl-4H-thiopyran-4-ylidene)-methyl!-2,4-dihydroxy-dihydroxide-cyclobutenediylium-bis{inner salt}. Other suitable infrared-absorbing dyes includethose described in U.S. Pat. No. 5,231,190 (and in the correspondingEuropean Application No. 92107574.3, Publication No. 516,985): inInternational Application No. PCT/US91/08695, Publication No. WO92/09661).

Thermal imaging medium 10 can be imaged by creating (in medium 10) athermal pattern according to the information imaged. Exposure sourcescapable of providing radiation which can be directed on to medium 10,and which can be convened by absorption into thermal energy, can beused. Gas discharge lamps, xenon lamps and lasers are examples of suchsources.

The exposure of medium 10 to radiation can be progressive orintermittent. For example, a medium as shown in FIG. 1 can be fastenedon to a rotating drum for exposure of the medium through sheet 12. Aradiation spot of high intensity, such as is emitted by a laser, can beused to expose the medium 10 in the direction of rotation of the drum,while the laser is moved slowly in a transverse direction across theweb, thereby to trace out a helical path. Laser drivers, designed tofire corresponding lasers, can be used to intermittently fire one ormore lasers in an imagewise and predetermined manner to thereby recordinformation according to an original to be imaged. As shown in FIG. 2, apattern of intense radiation can be directed on to medium 10 by exposureto a laser from the direction of the arrows 24, 24', 26 and 26', theareas between the respective pairs of arrows defining regions ofexposure.

If desired, an imaging medium of the invention can be imaged using amoving slit or stencils or masks, and by using a tube or other sourcewhich emits radiation continuously and which can be directedprogressively or intermittently on to medium 10. Thermographic copyingmethods can be used, if desired.

Preferably, a laser or combination of lasers is used to scan the mediumand record information in the form of very fine dots or pels.Semiconductor diode lasers and YAG lasers having power outputssufficient to stay within upper and lower exposure threshold values ofmedium 10 will be preferred. Useful lasers may have power outputs in therange of from about 40 to about 1000 milliwatts. An exposure thresholdvalue, as used herein, refers to a minimal power required to effect anexposure, while a maximum power output refers to a power level tolerableby the medium before "burn out" occurs. Lasers are particularlypreferred as exposing sources since medium 10 may be regarded as athreshold-type of film; i.e., it possesses high contrast and, if exposedbeyond a certain threshold value, will yield maximum density, whereas nodensity will be recorded below the threshold value. Especially preferredare lasers which are capable of providing a beam sufficiently fine toprovide images having resolution as fine as 4,000-10,000 dots per inch(160-400 dots per millimeter).

Locally applied heat, developed at or near the interface ofimage-forming layer 14 and heat-activatable zone or layer 12c can beintense (about 400° C.) and serves to effect imaging in the mannerdescribed above. Typically, the laser dwell time on each pixel will beless than one millisecond, and the temperature in exposed regions can bebetween about 100° C. and about 1000° C.

Apparatus and methodology for forming images from thermally actuatablemedia such as the medium 10 are described in detail in U.S. Pat. No.5,170,261 (and the corresponding International Application No.PCT/US91/06880, Publication No. WO 92/10053); and in U.S. Pat. No.5,221,971 (and the corresponding International Application No.PCT/US91/06892, Publication No. WO 92/10057).

The imagewise exposure of medium 10 to radiation creates in the mediumlatent images which are viewable upon separation of the sheets thereof(12 and 22) as shown in FIG. 2. Sheet 22 can comprise any of a varietyof plastic materials transmissive of actinic radiation used for thephotohardening of photohardenable adhesive layer 20. A transparentpolyester (e.g., poly(ethylene terephthalate)) sheet material ispreferred. In addition, sheet 22 will preferably be subcoated, or may becorona treated, to promote the adhesion thereto of photohardened anddurable layer 20. Preferably, each of sheets 12 and 22 will be flexiblepolymeric sheets.

The thermal imaging medium of the invention is especially suited to theproduction of hardcopy images produced by medical imaging equipment suchas X-ray equipment, CAT scan equipment, MR equipment, ultrasoundequipment and so forth. As stated in Neblette's Handbook of Photographyand Reprography, Seventh Edition, Edited by John M. Sturge, Van Nostrandand Reinhold Company, at pp. 558-559: "The most important sensitometricdifference between X-ray films and films for general photography is thecontrast. X-ray films are designed to produce high contrast because thedensity differences of the subject are usually low and increasing thesedifferences in the radiograph adds to its diagnostic value . . .Radiographs ordinarily contain densities ranging from 0.5 to over 3.0and are most effectively examined on an illuminator with adjustablelight intensity . . . Unless applied to a very limited density range theprinting of radiographs on photographic paper is ineffective because ofthe narrow range of density scale of papers." The medium of the presentinvention can be used to advantage in the production of medical imagesusing printing apparatus, as described in the aforementioned U.S.application Ser. No. 07/616,658, which is capable of providing a largenumber of gray scale levels.

The use of a high number of gray scale levels is most advantageous athigh densities inasmuch as human vision is most sensitive to gray scalechanges which occur at high density. Specifically, the human visualsystem is sensitive to relative change in luminance as a function ofdL/L where dL is the change in luminance and L is the average luminance.Thus, when the density is high, i.e., L is small, the sensitivity ishigh for a given dL whereas if the density is low, i.e., L is large,then the sensitivity is low for a given dL. Accordingly, the medium ofthe present invention is especially suited to utilization with equipmentcapable of providing small steps between gray scale levels at the highend of the gray scale, i.e., in the high contrast region of greatestvalue in diagnostic imaging. Further, it is desirable that the highdensity regions of the gray scale spectrum be rendered as accurately aspossible, since the eye is more sensitive to errors which occur in thatregion of the spectrum.

The medium of the present invention is especially suited to theproduction of high density images as image 10b, shown in FIG. 2. It hasbeen noted previously that separation of sheets 12 and 22 withoutexposure, i.e., in an unprinted state, provides a totally dense image incolorant material on sheet 22 (image 10b). The making of a copy entailsthe use of radiation to cause the image-forming colorant material to befirmly attached to web 12. Then, when sheets 12 and 22 are separated,the exposed regions will adhere to web 12 while unexposed regions willbe carried to sheet 22 and provide the desired high density image 10b.Since the high density image provided on sheet 22 is the result of"writing" on sheet 12 with a laser to firmly anchor to sheet 12 (andprevent removal to sheet 22) those portions of the colorant materialwhich are unwanted in image 10b, it will be seen that the amount oflaser actuation required to produce a high density image can be kept toa minimum. A method for providing a thermal image while keeping exposureto a minimum is disclosed and claimed in International Application No.PCT/US91/06898, Publication No. WO 92/09939.

If medium 10 were to be exposed in a manner to provide a high densityimage on sheet 12, it will be appreciated that the high density grayscale levels would be written on sheet 12 with a single laser at aninefficient scanning speed or by the interaction of a number of lasers,increasing the opportunity for tracking error. Because medical imagesare darker than picture photographs and tracking errors are more readilydetected in the high density portion of gray scale levels, a printingapparatus, using medium 10, would need to be complex and expensive toachieve a comparable level of accuracy in the production of a highdensity medical image on sheet 12 as can be achieved by exposing themedium for production of the high density image on sheet 22.

Since image 10b, by reason of its informational content, aesthetics orotherwise, will often be considered the principal image of the pair ofimages formed from medium 10, it may be desired that the thickness ofsheet 22 be considerably greater, and the sheet 22 thus more durable,than sheet 12. In addition, it will normally be beneficial from thestandpoints of exposure and energy requirements that sheet 12, throughwhich exposure is effected, be thinner than sheet 22. Asymmetry in sheetthickness may increase the tendency of the medium material to delaminateduring manufacturing or handling operations. Utilization ofphotohardenable adhesive layer 20 will be preferred in medium 10particularly to prevent delamination during manufacture of the medium.

If desired, further protection for the image 10b against abrasion andadded durability can be achieved by including an additional layer (notshown) of a thermoplastic material between image-forming layer 14 andsurface zone or layer 12c, which additional layer comprises a polymericlayer fracturable substantially along the exposure direction and whichprovides surface protective portions (over image portions 14b) forimproved durability of image 10b. A laminar thermal imaging mediumincluding a thermoplastic intermediate layer to provide surfaceprotection of an image prepared therefrom is disclosed and claimed inU.S. Pat. No. 5,155,003, and the corresponding International ApplicationNo. PCT/US91/08601, Publication No. WO 92/09442.

Alternatively, additional durability can be provided to image 10b bydepositing a protective polymeric overcoat layer thereon. A protectedimage and method therefor are disclosed and claimed in InternationalApplication No. PCT/US91/08345, Publication No. WO 92/09930. A preferredmethod for protecting an image using a siloxane-containing overcoatlayer is disclosed and claimed in application Ser. No. 08/065,345, filedJun. 25, 1993 and assigned to the same assignee as the presentapplication. Another preferred method for protecting an image using aprotective overcoat comprising a durable layer and a barrier layerresistant to the penetration of solvent is disclosed and claimed inapplication Ser. No. 08/118,882, filed Sep. 9, 1993 and assigned to thesame assignee as the present application.

The following Example is now given, though by way of illustration only,to show details of particularly preferred reagents, conditions andtechniques used in the method and imaging medium of the presentinvention. All parts, ratios and proportions, except where otherwiseindicated, are by weight.

EXAMPLE

On to a first sheet of poly(ethylene terephthalate) of 1.75 mil (44 μm)thickness (ICI Type 3284 film, available from ICI Americas, Inc.,Hopewell, Va.) were deposited the following layers in succession:

a 2.5 μm thick stress-absorbing layer of polyurethane (ICI XR-9619,available from ICI Resins US, Wilmington, Mass.);

a 0.9 μm thick heat-activatable layer of poly(styrene-co-acrylonitrile);

a 1 μm thick layer of carbon black pigment, polyvinyl alcohol (PVA),styrenated acrylate dispersing agent (Joncryl 67, available from JohnsonWax Company, Racine, Wis.) and 1,4-butanediol diglycidyl ether, atratios, respectively, of 5/1/0.5/0.18;

a 1 μm thick release layer comprising silica, PVA, styrenated acrylatelatex particles (Joncryl 87, available from Johnson Wax Company, Racine,Wis.), sodium salt of copolymer of maleic anhydride and vinyl methylether (Gantrez S-97, molecular weight approximately 100,000, availablefrom GAF Corporation), and ammonium perfluoroalkyl sulfonate surfactant(FC-120, available from Minnesota Mining and Manufacturing Corporation,St. Paul, Minn. 55144-1000), at ratios, respectively, of30/21/2/0.6/0.2; and

a 3.6 μm thick layer comprising a copolymer of vinylidene chloride andmethyl acrylate (Daran SL-158, available from Hampshire ChemicalCorporation, 55 Hayden Road, Lexington Mass. 02173) and ammoniumperfluoroalkyl sulfonate surfactant (the aforementioned FC-120) at 0.02%of the total volume of the coating solution.

To form the second adhesive layer, 10.5 parts of butyl acrylate, 87.7parts of butyl methacrylate and 1.8 parts by weight of acrylic acid werecopolymerized with 2,2'-azobis(2-methylpropanenitrile) to form acopolymer having a number average molecular weight of about 40,000. Acoating solution was prepared comprising 11.90 parts of this copolymer,2.82 parts of trimethylolpropane triacrylate (TMPTA, available asAgeflex TMPTA from CPS Chemical Company, Old Bridge, N.J. 08857), 0.007parts of 4-methoxyphenol (a free radical inhibitor), 1.14 parts of2,2-dimethoxy-2-phenylacetophenone (a photoinitiator, available asIrgacure 651 from Ciba-Geigy Corporation), 0.037 parts oftetrakis{methylene(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)}methane(an antioxidant, available as Irganox 1010 from Ciba-Geigy Corporation),0.037 parts of thiodiethylenebis(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate (an anti-oxidant,available as Irganox 1035 from Ciba-Geigy Corporation), and 58.28 partsof ethyl acetate solvent. This coating solution was coated on to 4 mil(101 μm) poly(ethylene terephthalate) film (ICI Type 526 anti-statictreated film, available from ICI Americas, Inc., Hopewell, Va.; thisfilm forms the second web 22 of the imaging medium 10) and dried in anoven at about 85° C. (185° F.) to a coating weight of about 9400 mg/m²to form a hardenable second adhesive layer 20 approximately 10 μm thick.

The first and second poly(ethylene terephthalate) sheets wereimmediately brought together with their adhesive layers in face-to-facecontact, the 4 mil sheet being in contact with a rotating steel drum. Arubber roll having a Durometer hardness of 70-80 was pressed against the1.75 mil sheet. The resulting web of laminar medium was then passed inline, approximately 90 seconds after lamination, under aradio-frequency-powered source of ultraviolet radiation, with the 4 milsheet facing, and at a distance of about 2.5 inches (6.4 cm) from, thesource (a Model DRS-111 Deco Ray Conveyorized Ultraviolet Curing System,sold by the aforementioned Fusion UV Curing Systems), which served tocure the second adhesive layer 20.

After curing, the web of imaging medium was passed through a slittingstation where edgewise trimming along both edges of the medium wasperformed in the machine direction. The resultant trimmed web was thenwound on to a take-up roll.

Individual sheets of imaging medium cut from the resultant roll wereimaged by laser exposure through the 1.75 mil sheet using high intensitysemiconductor lasers. In each case, the medium was fixed (clamped) to arotary drum with the 4 mil sheet facing the drum. The radiation ofsemiconductor lasers was directed through the 1.75 mil sheet in animagewise manner in response to a digital representation of an originalimage to be recorded in the medium. After exposure to the high-intensityradiation (by scanning of the imaging medium orthogonally to thedirection of drum rotation) and removal of the exposed imaging mediumfrom the drum, the two sheets of the imaging medium were separated toprovide a first image on the first, 1.75 mil sheet and a second (andcomplementary) image on the second, 4 mil sheet (the principal image).

We claim:
 1. A method of preparing a laminar thermal imaging mediumwhich comprises the steps of:providing a first element comprising afirst sheet transparent to image-forming radiation and having at least asurface zone or layer of polymeric material heat-activatable uponsubjection of the thermal imaging medium to brief and intense radiation,the first element carrying a layer of porous or particulateimage-forming substance having cohesivity in excess of its adhesivityfor the polymeric heat-activatable layer, and, on the opposed side ofthe layer of porous or particulate image-forming substance from thesurface zone or layer, a first layer of adhesive comprising ahalogen-containing polymer; providing a second element comprising asecond sheet carrying a second layer of adhesive; the second layer ofadhesive comprising a polymeric hardenable adhesive comprising amacromolecular organic binder having acidic groups, and aphotopolymerizable monomer; laminating the first and second elementstogether with the first and second layers of adhesive in contact withone another and with the first and second sheets outermost and forming aunitary laminar medium in which the hardenable adhesive remains in itsunhardened condition and serves to reduce the tendency for the unitarylaminar medium to delaminate on application of stresses to the medium;and subjecting the unitary laminar medium to actinic radiation effectiveto cause polymerization of the photopolymerizable monomer, thushardening the hardenable adhesive into a durable polymeric layer.
 2. Amethod according to claim 1 wherein the halogen-containing polymercomprises repeating units derived from vinylidene chloride.
 3. A methodaccording to claim 2 wherein the halogen-containing polymer comprises acopolymer of vinylidene chloride and an acrylate or methacrylate.
 4. Amethod according to claim 1 wherein the acidic groups of themacromolecular organic binder comprise carboxylic acid groups.
 5. Amethod according to claim 4 wherein the macromolecular organic binderhaving acidic groups comprises a polymer having repeating units derivedfrom acrylic or methacrylic acid.
 6. A method according to claim 4wherein the macromolecular organic binder having acidic groups comprisesa polymer having repeating units derived from β-carboxyethyl acrylate ormaleic acid.
 7. A method according to claim 5 wherein the macromolecularorganic binder comprises a copolymer of at least one acrylate ormethacrylate ester with at least one of acrylic and methacrylic acid. 8.A method according to claim 7 wherein the macromolecular organic bindercomprises a copolymer of:from 0 to about 20 parts by weight of butylacrylate; from about 70 to about 99 parts by weight of butylmethacrylate; and from about 1 to about 10 parts by weight of acrylicacid.
 9. A method according to claim 1 wherein the photopolymerizablemonomer comprises a di- or higher functional acrylate or methacrylate.